What Time Is the November 2021 Blood Moon? Lunar Eclipse to Make Beaver Moon Appear Red

On Thursday an almost complete lunar eclipse will turn November's full moon, the Beaver Moon, a blood-red color. The partial eclipse will be the longest eclipse for 580 years, according to Space.com, lasting 3 hours, 28 minutes, and 23 seconds.

The title of "Blood Moon" is normally reserved for total lunar eclipses. Though not a total eclipse, because, according to NASA, 99.1 percent of the moon will be within the darkest part of Earth's shadow, this partial eclipse is almost total enough to be considered a Blood Moon.

A lunar eclipse happens when the moon, the sun, and the Earth align, with the Earth in between the moon and the sun. This leads to the shadow of the Earth falling on the moon. When the moon is completely in the darkest part of Earth's shadow, the umbra, it is a total eclipse.

Partial lunar eclipses happen because the moon, the sun, and our planet aren't always perfectly aligned, meaning that sometimes only part the moon falls within Earth's shadow. Or it falls in the less dark part of Earth's shadow, the penumbra, in something called a penumbral eclipse.

Thursday's eclipse will begin at 1:02 a.m. EDT when the moon slips into Earth's penumbra, the outer part of the shadow. By 3:45 a.m. it will have passed into the inner, darker shadow, the umbra, by around 95 percent.

The peak of the eclipse will happen at 4:03 a.m., the best time to see the Beaver Moon as a "Blood Moon" as this will be the point when the red coloration is most visible.

Lunar Eclipse
A diagram showing a lunar eclipse. November's eclipse won't be total, but as 99.1 percent of the moon will be in Earth's shadow, it is considered "almost total." NASA

Because Blood Moons are usually associated with total lunar eclipses, we can easily predict how many Blood Moons Earth will experience each year For instance, in 2022 the Earth will see two total lunar eclipses, one on May 15, the other on November 7.

This is actually below the average of lunar eclipses for a year, which is three. Lunar eclipses only happen when the moon is full. This means though it only takes 29.5 days to orbit Earth and complete a full cycle from full moon to full moon, because the moon's orbit is angled at five degrees, it often slips above or below Earth's shadow.

While a Blood Moon may have had supernatural significance to our ancestors, thanks to science we now know the physical process that causes the moon to take a reddish hue during an eclipse.

The process is actually related to the reason the sky is blue, known as Rayleigh scattering.

As Isaac Newton discovered, light from the sun is made up of all different colors of light. These colors take a specific order, red, orange, yellow, green, blue, indigo, and violet, because of the size of their wavelength and their frequency. Red light has a long wavelength and a low frequency, and blue light has a short wavelength and a high frequency.

This means that when the sun is directly overhead red and blue photons, and the individual particles that comprise light stream through our atmosphere, the red photons are able to reach the ground pretty much unhindered.

The blue photons, however, because of their short wavelength, hit atoms in the upper atmosphere and are bounced off them, or scattered. This means before a blue photon hits your eye, it has traveled the sky. That's why the sky appears blue.

The situation is different when the sun is on the horizon. Because its light then has much more atmosphere to travel through, the red photons begin to be scattered. That's why the sky appears red at sunset sometimes.

In the case of a lunar eclipse, because the Earth is in between the sun and the moon, photons from the sun have to traverse Earth's atmosphere to hit the moon and be reflected. This means there is more opportunity for red photons to be scattered.

Astronomers don't really like the term "Blood Moon" to describe lunar eclipses. This is because the moon is, of course, not actually "turning red." Nothing is actually happening to the moon, it's just the effect of our atmosphere.

Also, how "red" the moon will appear depends strongly on how many clouds and how much dust is in the Earth's atmosphere. That means that the "Blood Moon" effect is often very subtle and barely noticeable.

That doesn't mean the partial eclipse on November 19 won't be worth watching, however. The almost total eclipse of the Beaver Moon, which takes its name from the fact beavers prepare for winter as it occurs, will be visible in various regions of the globe, including North America, eastern Australia, New Zealand, and Japan.

Blood Moon
An image of a Blood Moon, the name usually reserved for a total solar eclipse. November's Beaver Moon will be eclipsed enough to be considered a Blood Moon. NASA